The FBI revealed Friday it was reviewing a new batch of emails that "appear to be pertinent" to its previous investigation into Hillary Clinton's use of a private server while she was secretary of state.

"In connection with an unrelated case, the FBI has learned of the existence of emails that appear to be pertinent to the investigation," FBI director James Comey said in a letter to the Senate Judiciary Committee. "I agreed that the FBI should take appropriate investigative steps designed to allow investigators to review these emails to determine whether they contain classified information, as well as to assess their importance to our investigation."

A senior law enforcement official told NBC News Friday that the Comey letter was sent to the Hill "out of an abundance of caution" and to be extra-thorough.

The official said the emails were discovered "on another device."

There's no indication, the official said, that Clinton, her campaign or the State Department was withholding information. But the emails were not held by someone who was investigated in the Clinton email case, the official said.

The politically explosive revelation, which came just 11 days before Election Day, appeared to catch the Clinton campaign by surprise.

"No idea," a top Clinton campaign spokesperson told NBC News.

Tim Kaine, Clinton's vice presidential running mate, who was at an early voting site in Tallahassee on Friday told reporters "gotta read a little more, gotta read a little more" before continuing on.

GOP nominee Donald Trump, at a campaign event in New Hampshire, praised the FBI for having "the courage to right the horrible mistake they made," an apparent reference to the agency's decision earlier this year to not press criminal charges against Clinton.

"I need to open with a very critical breaking news announcement," Trump said to cheers that at times drowned out his remarks. "The FBI has just sent a letter to Congress informing them that they have discovered new emails pertaining to the former Secretary of State Hillary Clinton's investigation."

"This was a grave miscarriage of justice," he said, "it is about to be corrected."